Large capacity acid or base generator apparatus

ABSTRACT

Method and apparatus for generating an acid or base, e.g. for chromatographic analysis of anions. For generating a base the method includes the steps of providing a cation source in a cation source reservoir, flowing an aqueous liquid stream through a base generation chamber separated from the cation source reservoir by a barrier (e.g. a charged membrane) substantially preventing liquid flow while providing a cation transport bridge, applying an electric potential between an anode cation source reservoir and a cathode in the base generation chamber to electrolytically generate hydroxide ions therein and to cause cations in the cation source reservoir to electromigrate and to be transported across the barrier toward the cathode to combine with the transported cations to form cation hydroxide, and removing the cation hydroxide in an aqueous liquid stream as an effluent from the first base generation chamber. Suitable cation sources include a salt solution, a cation hydroxide solution or cation exchange resin.

CROSS-REFERENCE TO RELATED APPLICATION

[0001] Reference is made to co-pending H. Small, et al. U.S. patentapplication Ser. No. 08/783,317, filed Jan. 15, 1997.

BACKGROUND OF THE INVENTION

[0002] The present invention relates to a large capacity apparatus forgenerating a high purity acid or base particularly for use as achromatography eluent, and to a method of using the apparatus.

[0003] In liquid chromatography, a sample containing a number ofcomponents to be separated is directed through a chromatographyseparator, typically an ion exchange resin bed. The components areseparated on elution from the bed in a solution of eluent. One effectiveform of liquid chromatography is referred to as ion chromatography. Inthis known technique, ions to be detected in a sample solution aredirected through the separator using an eluent containing an acid orbase and thereafter to a suppressor, followed by detection, typically byan electrical conductivity detector. In the suppressor, the electricalconductivity of the electrolyte is suppressed but not that of theseparated ions so the latter may be detected by the conductivitydetector. This technique is described in detail in U.S. Pat. Nos.3,897,213, 3,920,397, 3,925,019 and 3,956,559.

[0004] There is a general need for a convenient source of high purityacid or base for use as an eluent for liquid chromatography and,particularly, for ion chromatography. In one technique, described inU.S. Pat. No. 5,045,204, an impure acid or base is purified in an eluentgenerator while flowing through a source channel along a permselectiveion exchange membrane which separates the source channel from a productchannel. The membrane allows selective passage of cations or anions. Anelectrical potential is applied between the source channel and theproduct channel so that the anions or cations of the acid or base passfrom the former to the latter to generate therein a base or acid withelectrolytically generated hydroxide ions or hydronium ions,respectively. This system requires an aqueous stream of acid or base asa starting source or reservoir.

[0005] There is a particular need for a pure source of acid or basewhich can be generated at selected concentrations solely from an ionexchange bed without the necessity of an independent reservoir of anacid or base starting aqueous stream. There is a further need for such asystem which can be continuously regenerated. Such need exists inchromatography, and specifically ion chromatography, as well as otheranalytical applications using acid or base such as in titration, flowinjection analysis and the like.

SUMMARY OF THE INVENTION

[0006] In copending application Ser. No. 08/783,317, filed Jan. 15,1997, a method and apparatus is described for generating acid or base inan aqueous stream, such as water alone or in combination with additives(e.g., ones which react with the acid or base or with the sample). Thesystem provides an excellent source of high purity acid or base for useas an eluent for chromatography and, particularly, ion chromatography.The present system is an improvement over the one described in thecopending application.

[0007] Referring first to the present system in which a base isgenerated e.g. for chromatographic analysis of anions, the methodincludes the steps of:

[0008] (a) providing a cation source in a cation source reservoir,

[0009] (b) flowing an aqueous liquid stream through a base generationchamber separated from the cation source reservoir by a barriersubstantially preventing liquid flow while providing a cation transportbridge,

[0010] (c) applying an electric potential between an anode in electricalcommunication with said cation source reservoir and a cathode inelectrical communication with the base generation chamber toelectrolytically generate hydroxide ions in the base generation chamberand to cause cations in the cation source reservoir to electromigratetoward said first barrier and to be transported across the barriertoward the cathode to combine with the transported cations to formcation hydroxide, and

[0011] (d) removing the cation hydroxide in an aqueous liquid stream asan effluent from the first base generation chamber.

[0012] Suitable cation sources include a salt solution or a cationhydroxide solution which can be supplied to the cation source reservoirby pumping from a remote reservoir. The solution can be recycled to theremote reservoir. Also, the cation source may comprise a cation exchangebed, e.g., resin particles in a stationary bed or suspended in anaqueous liquid, alone or in combination with the salt solution.

[0013] The method may also be used for generating an acid, e.g. for useas an eluent for chromatographic analysis of cations by reversing thecharges of the ion source, the barrier, the electrical potential and anyother charged components of the system.

[0014] Another embodiment of the invention is an apparatus forgenerating an acid or base including:

[0015] (a) an ion source reservoir of either anions or cations,

[0016] (b) an acid or base generation chamber having inlet and outletports,

[0017] (c) a first barrier between the ion source reservoir and the acidor base generation chamber, substantially preventing liquid flow whileproviding an ion transport bridge for only ions of one charge, positiveor negative,

[0018] (d) a first electrode in electrical communication with the ionsource reservoir,

[0019] (e) a second electrode in electrical communication with the firstacid or base generation chamber, and

[0020] (f) an aqueous liquid source in fluid communication with the acidor base generation chamber inlet port.

[0021] The apparatus can be used to supply the generated acid or base toa chromatography system or any other analytical system which uses a highpurity acid or base.

BRIEF DESCRIPTION OF THE DRAWINGS

[0022] FIGS. 1-8 and 10-12 are schematic representations of apparatusaccording to the present invention.

[0023]FIG. 9 is an on-line high pressure gas removal device for use inthe present invention.

[0024] FIGS. 13-29 are graphical representations of experimental resultsusing the present base or acid generator system.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0025] The system is applicable to the generation of eluent for liquidchromatography forms other than ion chromatography. For example, it isapplicable to liquid chromatography using an ultraviolet (UV) detector.The eluent may be in a form (e.g. salt) other than a pure acid or base.Thus, the term “aqueous stream” includes pure water or water with suchadditives. Also, the terms “eluent comprising a base”, “eluentcomprising an acid”, an “acid” or a “base” mean an aqueous streamincluding acid or base generated according to the invention regardlessof the form it takes on mixing with other reagents present in theaqueous stream. As used herein, the term “cation” excludes hydronium ionand the term “anion” excludes hydroxide ion. The system is alsoapplicable to other non-chromatographic analytical systems which use ahigh purity acid or base.

[0026] The copending application uses some of the same principles as thepresent invention and its disclosure is incorporated by reference. Suchdisclosure includes a high purity solution of acid or baseelectrochemically generated by passing deionized water through anelectrically polarized bed of ion exchange resin in the desired ionicform placed between two electrodes. For example, in the generation of aKOH solution, deionized water is pumped through a column packed with acation exchange resin in K⁺ form, and a DC voltage is applied betweenthe anode at the column inlet and the cathode at the column outlet. Theelectrochemical reaction at the anode generates H⁺ ions by splittingwater. Under the influence of the electrical field, H⁺ ionselectromigrate into the resin bed to displace K⁺ ions, which in turnmigrate downstream through the resin bed and combine with OH⁻ ionsgenerated at the cathode to produce KOH. The concentration of KOHgenerated is determined by the electrical current applied and the flowrate of the deionized water through the column. Similarly, a high purityacid (e.g., methanesulfonic acid) solution can be generated using ageneration column containing an anion exchange resin in the desiredionic form.

[0027] The acid or base generation column described above is anattractive source of high purity eluent for ion and liquidchromatography for a number of reasons. For example, chromatographicseparations can be conveniently performed using only deionized water asthe carrier. Since acid or base is generated on-line, the need ofoften-tedious, off-line preparation of eluents can be eliminated.Second, the eluent strength (the concentration of acid or base) can becontrolled precisely and conveniently by controlling the electricalcurrent applied to the acid or base generation column and the flow rate.Third, gradient chromatographic separations can be accomplished withcurrent gradients and a less costly isocratic pump instead of using amore expensive gradient pump. Fourth, the use of an acid or basegeneration column can improve the performance of chromatographicmethods, since the eluent generated on-line can be free of contaminantsthat are often introduced if it is prepared off-line by conventionalmeans. For example, the presence of carbonate in hydroxide eluent due tosorption of carbon dioxide from air often seriously compromises theperformance of an ion chromatography method; this problem will beeliminated by using the high purity hydroxide eluent generated on-line.Fifth, the reliability of the chromatography pumping system can beimproved, the lifetime of pump seal can be extended significantly sincethe pump is used to pump deionized water instead of more corrosive acidor base solution. These same advantages and principles apply to thepresent invention. In addition, the present invention retains theadvantages of the acid or base generation column, and provides asignificant improvement in the generation of high purity acid or basesolutions for an extended period of time for ion and liquidchromatography, and other applications.

[0028] The method and apparatus for generation of acid or base accordingto the present invention will first be described to supply eluent, e.g.,for ion chromatography. Although applicable to anion or cation analysis,the system will be described for generation of a base suitable for useas an eluent in the analysis of anions on an ion exchange resin packedbed form. In this instance, the cation exchange bed generates a basesuch as an alkali metal hydroxide, typically sodium or potassium. Foranalysis of cations, the eluent generated is an acid such asmethanesulfonic acid. The system will first be described for thegeneration of KOH as the base.

[0029]FIG. 1 schematically illustrates a general form of a largecapacity base (KOH) generator form according to the present invention.The apparatus includes cation (K⁺) ion source reservoir 10. As will beexplained in more detail below, the cation source may be acation-containing solution such as a salt solution or a cation hydroxidesolution. Alternatively, the cation source may be a cation exchange bedincluding exchangeable cations of the type which form a cationhydroxide. The bed may be formed of ion exchange resin particles in afixed or stationary bed or suspended particles in an aqueous liquid. Agas vent may be provided in reservoir 10 to vent oxygen generatedtherein as described hereinafter.

[0030] Base generation chamber 12 is separated from the ion sourcereservoir 10 by a barrier 14, suitably in the form of a chargedperm-selective membrane described below. Charged barrier 14substantially prevents liquid flow while providing an ion transportbridge for cations from the ion source reservoir 10 to base generationchamber 12. As used herein, the term “barrier” refers to the chargedmaterial (e.g. membrane) separating reservoir 10 and chamber 12 whichpermits ion flow but blocks liquid flow, alone or in combination with anappropriate flow-through housing in which the barrier is mountedtransverse to flow across the entire flow path.

[0031] The charged barrier 14 should be of sufficient thickness towithstand the pressures in chamber 12. For example, if chamber 12 is online with a chromatography system, such pressures may be on the order of1,000 to 3,000 psi. When using a membrane as barrier 14, it is suitablyconfigured of circular cross-section within a cylindrical external shortcolumn. Typical dimensions for the membrane are about 4-6 mm diameterand 1-3 mm in length. The barrier can be fabricated by stacking multipledisks of cation membranes together within the cylindrical column.Alternatively, barrier 14 can be prepared from a single ion exchangemembrane of appropriate thickness or a block or rod of appropriate ionexchange material which permits passage of the potassium but not of theliquid.

[0032] An anode 16 is disposed in electrical contact with, andpreferably within, cation source reservoir 10 and a cathode 18 isdisposed in electrical contact with, and preferably within, basegeneration chamber 12. A suitable DC power supply 20 connects the anodeand the cathode. Also, there is a continuous electrical path from anode16 through barrier 14 to cathode 18. Aqueous stream 20, suitablydeionize water, flows through an inlet port, not shown, in basegeneration chamber 12. KOH is generated in base generation chamber 12and flows out of outlet port, not shown. A cation exchange resin bed 19(e.g. in K⁺ form) can be packed in chamber 12 in contact with barrier 14and cathode 18 to provide good electrical contact therebetween. Asillustrated, the flow of aqueous stream 20 is toward cathode 18.However, if desired, the flow may be in the opposite direction.

[0033] For the production of pure base (e.g. KOH), high-purity deionizedwater from source 21 is pumped to generation chamber 12. Water splittingtakes place at both electrodes. The anode reaction in reservoir 10 is asfollows:

H₂O−2e⁻→2H⁺+½O₂   (1)

[0034] During this reaction, hydronium ions are produced in reservoir 10for the resin form of the invention, the hydronium ions pass into thecation exchange resin by electromigration displacing the exchangeablecations (e.g. K⁺ ions) ahead of them. This displacement takes placealong the length of the bed and the K⁺ ions pass through barrier 14 intochamber 12 eventually leading to production of base (KOH) in the flowingaqueous stream in generation chamber 12. The hydroxide ions are producedin the following cathodic reaction.

2H₂O+2e⁻→2OH⁻+H₂   (2)

[0035] In one form of reservoir 10, the cation source is acation-containing solution, suitably either a salt solution or a cationhydroxide solution (e.g. KOH). If a salt solution is used, it ispreferably of a weakly acidic anion salt such as K₂HPO₄ to bind thehydronium ions produced at the anode. In this manner, K⁺ is the primaryion passing through barrier 14, thereby minimizing the flow of H⁺ ions.The hydronium ion generation in the reservoir provides electricalneutrality to the solution in the reservoir as the K⁺ ions are drivenacross the barrier.

[0036] Another embodiment of the invention is illustrated in FIG. 2.This device is specifically adapted for use with an ion exchange resinform of cation source in reservoir 10. Because of the similar componentsin FIGS. 1 and 2, like parts will be designated with like numbers. Theillustrated reservoir 10 is suitably in the form of a solid horizontalhollow cylinder 10 a with inlet and outlet walls 10 b and 10 c,respectively, and packed with cation exchange resin in K⁺ form.Alternative shapes, e.g. rectangular, of reservoir 10 may be used. Anaqueous stream, suitably dionized water, is pumped through an inletport, not shown, into reservoir 10. Similarly, a preferred housing forchamber 12 is a cylindrical column defining a cylindrical chamber. Thus,the terms “chamber” and “column” will be used interchangeably forchamber 12. Anode 16 is illustrated as a perforate disk disposed at theinlet side of reservoir 10 adjacent inlet wall 10 b. Flow-through cationexchange resin bed 24 is suitably of similar ion exchange and flowcharacteristics to a chromatographic separation bed.

[0037] A preferred form of ion exchange resin bed in reservoir 10 is a“dual-bed” including a long section 24 a of a strongly acidic cationexchange resin (e.g. a sulfonated resin such as sold under thetrademarks Dowex 50WX8 resin or Dionex ASC resin) in K⁺ form adjacent atthe line X-X to a shorter section of a weakly acidic cation exchangeresin (e.g. a carboxylate resin such as sold under the trademarks DionexCS12A resin or Bio-Rex 70 resin) in K⁺ form downstream at its outletend. As used herein, “weakly acidic” anion means an anion with an aciddissociation constant (pKa) of greater than 3.0 and “strongly acidicanion” means an anion with a pKa less than about 3.0. Preferably thestrongly acidic section 24 a is at least about 10 percent of the lengthor volume of reservoir 10 and more preferably at least about 90 percentof the length or volume. Alternatively, if desired, the entire bed 24may be formed of strongly acidic cation exchange resin.

[0038] The dual-bed approach increases the useful capacity of a KOHgenerator column. Once H⁺ ions reach the bed of the weakly acidic resin,migration of H⁺ through the resin bed is significantly slowed downbecause of its higher affinity to the weakly acidic functional groups.On the other hand, the migration of K⁺ ions through the resin bed is notsignificantly reduced.

[0039] Therefore, more K⁺ ions are able to reach the cathode to form KOHbefore the arrival of H⁺ ions at the cathode, and thus the usefulcapacity of the KOH generator column is increased. In the dual-bed onceH⁺ ions reach the weakly acidic resin bed, the applied voltage needed tomaintain the constant current will increase due to the development ofthe less conductive protonated zone in the weakly acidic resin bed.

[0040] One function of barrier 14 is to permit use of a very largereservoir 10 (e.g. 1-2 liters) supplying K⁺ ions to generation chamber12. This large capacity reservoir permits a long term supply of K⁺ ions.By way of example, a typical KOH generation chamber may have a volume onthe order of less than 100 μL and more typically from 100 μL to 1,000μL. Suitable dimensions for a cylindrical shape are 4-7 mm ID and 10-50mm in length. This facilitates use on line in a chromatography system.In contrast, reservoir 10 may be many times larger than the volume ofthe generation chamber 12. For example, the ratio between reservoir 10and chamber 12 may be at least 5:1 to 10:1 or 20:1 or even higher.

[0041] Another function of barrier 14 is that it provides a highpressure physical barrier that insulates the relatively low pressure K⁺ion supply reservoir 10 from the generation chamber 12 which is ofsubstantially high pressure when it is on line with a high pressurechromatography system. For example, even a very low pressurechromatography system would be pressurized to at least about 50 psi.Assuming the reservoir's atmospheric pressure (14.7 psi) the pressuremaintained in the base generation chamber 12 is at least about threetimes the pressure maintained in reservoir 10. This isolation isparticularly useful when that pressure ratio is at least about 2:1 andis even more so when the ratio is much higher, for example at leastabout 5:1 to at least about 10:1 to 100:1 or higher.

[0042] Because it is operated under low pressure, a large K⁺ ion supplycolumn can be prepared and operated safely without demanding pressureconstraint. A large K⁺ ion supply column can contain a sufficient amountof cation exchange resin in K⁺ form to generate KOH over an extendedperiod of time. For example, a 10-cm ID×20-cm length K⁺ ion supplycolumn has an internal volume of 1570 mL and can contain 2670 meq of K⁺ions (calculated using the resin capacity of 1.7 meg/mL). If the KOHgenerator column is used to generate 20 mM KOH at 1.0 mL/min, itstheoretical capacity is 2225 hours, and an actual useful time isexpected to be more than 1300 hours, assuming 60 percent of the total K⁺ion capacity is ultimately utilized for the generation of KOH.

[0043] To step down from the large volume reservoir 10 to the smallersize base generation chamber 12, an adapter section in the form ofhollow cylindrical column 26 packed with cation exchange resin 28 may bedisposed in open communication with column 10 through an opening in theend wall 10 c of reservoir 10. Barrier 14 is disposed between cylinder26 and generation chamber 12. A suitable configuration of barrier 14 isa hollow cylinder transverse to cylinder 26 with a barrier disk (e.g.permselective membrane) across the flow path therebetween. Generationchamber 12 also is suitably is in the form of a hollow cylinder.

[0044] Barrier 14 is suitably in the form of a stack of cation exchangemembranes or a plug which prevents any significant liquid flow butpermits transport of the K⁺ ions into chamber 12. A suitable form ofmembrane is supplied by Membrane International of Glenrock, N.J.(designated CMI-7000 cation exchange membrane). As illustrated, cathode18 is a porous disk disposed adjacent to and coextensive with the endwall at the exit of chamber 14. As in the embodiment of FIG. 1, water issupplied to an inlet port of chamber 12. The KOH generated near cathode18 exits from the outlet of chamber 12. This is advantageous as the H₂gas generated at the cathode is readily swept out of chamber 12.

[0045] Anode 16 and cathode 18 disposed in reservoir 10 and generationchamber 12, respectively, can take the different forms such as porousdisks, frits, rings, screens, sheets, and probes so long as they providegood contact (preferably direct contact) with the ion source or ionexchange resin. For example, the anode is preferably in direct contactwith the ion exchange resin, if used, or with the solution in thereservoir if no ion exchange resin is used. Similarly, the cathodeshould be in direct contact with the ion exchange resin when used in thegeneration chamber. The electrode may also be formed by crumpling andforming a length of fine platinum wire to form a roughly disk-shapedobject that allows easy flow through the structure. The electrodes arepreferably made of inert material, such as platinum. In the embodimentsdescribed above, it is preferable that the electrodes be placed in aregion near the outlet of generation chamber 12, although otherlocations may be used as well.

[0046] In another form of the electrodes, not shown, a thin inertelectrically conductive screen is wrapped partially or totally around abed of ion exchange resin in chamber 12 in a case-like configuration.This electrode design provides good contact between the cation exchangeresin and the electrode surface, thus lowering the device operatingvoltage. Thus, higher currents can be applied to generate higherconcentrations without being limited by possible excessive heating.

[0047] In general, the method of the present invention using theembodiment of FIG. 2 is performed as follows. The cation source isprovided by the combination of cation exchange resin 24 in reservoir 12and cation exchange resin 28 in column 26. The H⁺ ion formed near anode12 drives the K⁺ ions through the resin until they transport acrossbarrier 14. The H⁺ ions produce electrical neutrality to reservoir 10.The K⁺ ions travel across barrier 14 into chamber 12 towards cathode 18and combines with the hydroxide ions formed at the cathode to form KOH.The aqueous stream flowing through base generation chamber 12 carriesthe KOH in solution for subsequent use in the analytical system.

[0048] When using a packed ion exchange bed in reservoir 10 orgeneration chamber 12, the higher the cross-linking of a resin thehigher its capacity (expressed as milliequivalents per ml. of column);therefore, higher cross-linked resins give more compact generators. Thisis desirable. However, the higher the cross-linking of a resin, the lessit deforms when packed in a column. Some deformation is desirable inthat it improves the area of contact between resin beads thus loweringthe electrical resistance of the packed bed. Lower resistance means thata particular level of current may be attained at a lower appliedvoltage; this, in turn, leads to less heating of the bed while carryingcurrent, a desirable feature.

[0049] Bead deformation is favored by lowering the degree of crosslinking. But, resin of very low cross-linking (say 1 to 2%) is sodeformable that at certain flow rates the deformation can lead toundesirably high pressure across the bed. In summary, a wide range ofcross-linking can be used. Resins of moderate cross-linkage are to bepreferred, typically in the range of 4 to 16% divinyl benzene forstyrene divinyl benzene polymer beads.

[0050] Other forms of ion exchange beds can be used such as a porouscontinuous structure with sufficient porosity to permit flow of anaqueous stream at a sufficient rate for use as an eluent forchromatography without undue pressure drop and with sufficient ionexchange capacity to form a conductive bridge of cations or anionsbetween the electrodes. One form of structure is a porous matrix or asponge-like material with a porosity of about 10 to 50% permitting aflow rate of about 0.1 to 3 ml/min without excessive pressure drop.Another suitable form is a roll of ion exchange film (e.g. in aconfiguration of such a roll on a spindle disposed parallel to liquidflow). Electrodes would be placed at each end of the roll which could betextured to provide an adequate void channel.

[0051] The aqueous stream flowing through chamber 12 may be high-puritydeionized water. However, for use in some forms of chromatography, itmay be desirable to modify the source with an additive which reacts withthe base generated in electrode chamber 12 to produce eluents of varyingpotency. For the production of base, some well known additives include asource of carbonic acid, phenol, cyanophenol, and the like. (For theproduction of acid, such additives include m-phenylene diamine,pyridine, lysine and amino propionic acid.)

[0052] It is preferable to control the concentration of base produced inbase generation chamber 12. To do so, the current, directly related toconcentration, is controlled. A feed-back loop may be provided to assuresufficient voltage to deliver the predetermined current. Thus, thecurrent is monitored when the resistance changes, and the potential iscorrespondingly changed by the feed-back loop. Therefore, the voltage isa slave to the reading of the current. Thus, it is preferable to supplya variable output potential system of this type (e.g., sold under thedesignation Electrophoresis Power Supply EPS 600 by Pharmacia Biotechand Model 220 Programmable Current Source by Keithley).

[0053] The current (voltage) requirements of a generator depend on (a)the eluent strength required; (b) the diameter of the column; (c) thelength of the column; (d) the electrical resistance of the resin; and(e) the flow rate of the aqueous phase.

[0054]FIG. 3 illustrates another embodiment of the invention. In thisinstance, no ion exchange resin is used in reservoir 10. Instead, asolution of a potassium salt such as K₂HPO₄ is employed. Alternatively,for specific applications, KOH may be used. The potassium salt solutionmay be used in combination with a cation exchange resin in K⁺ formeither in a fixed resin bed or in a bed in which the resin particles aresuspended in the solution. The concentration of K⁺ ions in solution ispreferable about 1 to 2 M or higher so that there is a sufficient amountof K⁺ ions for the generation of KOH over an extended time. However, ifdesired, the potassium salt solution containing K⁺ ions at lowerconcentrations (e.g. 0.1 to 0.5 M) can be used for specificapplications. It is preferable that the anion of the potassium salt notbe oxidized by the anode. It is preferable to use a potassium weaklyacidic anion (e.g., HPO₄ ²⁻ or CO₃ ²⁻) with an acid dissociationconstant (pK_(a)) of 5 or higher so that the concentration of free H⁺ions in the solution is kept lower than 0.1 mM. H⁺ ions, like K⁺ ions,can migrate across barrier 14 into generation chamber 12. If such H⁺migration occurs in significant amounts, the direct linear relationshipbetween the applied current and the concentration of KOH generated canbe lost because H⁺ ions can be combined with OH⁻ ions generated at thecathode to form water and thus the performance of the system can becompromised. By using the K₂HPO₄ salt, the following reaction occursusing H⁺ generated at anode 16 in equation (1) above.

2H⁺+2HPO₄ ²⁻=2H₂PO₄ ⁻  (3)

[0055] As in the embodiments of FIGS. 1 and 2, an aqueous stream ispumped through the generation chamber at 12 and a DC voltage is appliedbetween anode 16 and cathode 18. K⁺ ions migrate from reservoir 10 intogeneration chamber 12 through barrier 14 in the same manner describedabove. Also, as set out above, barrier 14 provides a high-pressurephysical barrier that prevents liquid leakage and diffusion of any ionsfrom reservoir 10 into generation chamber 12.

[0056] One advantage of this embodiment in which a solution withoutresin is used in reservoir 10 is that the potassium salt (e.g., K₂HPO₄)is a less expensive source of K⁺ ion than ion exchange resin withexchangeable K⁺ ions. Also, it is easier to replenish the reservoir witha fresh source of potassium salt. By way of example, in the embodimentof FIG. 3 using a one liter reservoir filled with 2.0 M K₂HPO₄ as atheoretical capacity of 4,000 meq K⁺ ions to generate 20 mM KOH at 1.0mL/min, the device will have a useful lifetime of 2500 hours, assuming a75 percent consumption of K⁺ ions in its K⁺ ion supply reservoir beforereplacing the salt solution.

[0057]FIG. 4 illustrates a flow-through strongly acidic cation exchangeresin bed 30 in K⁺ form disposed in reservoir 10. Anode 12 is suitablyin the form of a perforated platinum electrode at its outlet andadjacent an outlet port, not shown. Generation chamber 12 is separatedfrom reservoir 10 by barrier 14 of the type described above. In thisinstance, cation solution in the form of the potassium salt (e.g., 2.0 MK₂HPO₄) is continuously pumped by a pump 34 to a reservoir 10 at adesired rate (e.g. about 0.1 to 2.0 mL/min). The same principlesdescribed above with respect to concentration of the potassium salt andthe type of salt applied to this embodiment as well. Similarly, the sameflows and reactions occur in generator 12.

[0058] Continuous pumping of the potassium salt solution leads to acontinuous supply of K⁺ ions until the solution of salt in the remotereservoir is consumed.

[0059] In one embodiment illustrated in FIG. 4, the potassium saltsolution is recycled in recycle line 36 from the outlet of reservoir 10to the inlet of remote reservoir 32. The system can be operated untilthe concentration of K⁺ ions in remote reservoir 32 has been decreasedto a level insufficient to consistently generate KOH at the desiredconcentration. Then the device can be replenished by replacing thepotassium salt solution in the remote reservoir 32. Alternatively, inthe non-recycle mode, the solution exiting reservoir 10 flows to wasteas illustrated by dotted line 38. The flow rate of the potassium saltsolution can be slightly adjusted (e.g., about 0.005 to 0.050 mL/min) toprovide a sufficient supply of K⁺ ions to generate KOH at the desiredconcentration. Similarly, the device is replenished by filling theremote reservoir with potassium salt solution when the concentration hasdropped below the desired level.

[0060] In another embodiment of the invention, not shown, ion exchangeresin 30 may be eliminated from reservoir 10 so reservoir 10 is filledwith salt solution flowing from a remote reservoir 32. Otherwise thesystem is identical to the one described above.

[0061] Referring to FIG. 5, another embodiment of the invention isillustrated including multiple generation chambers 12 a, 12 b, and 12 cconnected in series, each one including its own cathodes 18 a, 18 b, and18 c. Generation chambers 12 a, 12 b, and 12 c are connected toreservoir 10 by barriers 14 a, 14 b, and 14 c as described above. Thedifference is that there are smaller generation chambers and smallerbarriers. By way of example, if each generation chamber is applied witha current of 80 mA to generate 25 mM of KOH at 2.0 mL/min the KOHgenerator with three generation chambers is capable of producing about75 mM of KOH at 2.0 mL/min. Additional KOH generation chambers may alsobe employed. An advantage of using two or more generation chambers isthat the operating voltage of the system may be lowered because theapplied current used to generate KOHs distributed among the generationchambers. Thus higher currents may be applied to generate the base ofhigher concentrations without being limited by potentially excessiveheating.

[0062] In another embodiment, not shown, two or more cathodes may bedisposed in a generation chamber 12, preferably spaced along the lengthof the chamber in the direction of aqueous liquid flow, e.g. near theinlet and outlet. This can serve to lower the electrical resistance ofthe chamber and thus the operating voltage of the system.

[0063] Referring to FIG. 6, another embodiment of the invention isillustrated using a single generation chamber 12 and two barriers 14 aand 14 b interconnecting chamber 12 and reservoir 10. Use of multiplebarriers can reduce the device operating voltage. Therefore thegeneration chamber 12 can be supplied with higher currents to generateKOH at higher concentrations without being limited by potentiallyexcessive heating. Another advantage in the use of multiple barriers isthat flexible membranes of smaller areas have better resistance tobursting than larger area membranes.

[0064] Referring to FIG. 7, use of the KOH generator of the presentinvention is schematically illustrated on-line in an ion chromatographyor liquid chromatography system. Water from source 40 is pumped by pump42 through the generation chamber of the large capacity KOH generator 44with an anode in the cation source reservoir and a cathode in thegeneration chamber connected to a power supply 45, as described above.Generator 44 is on-line with a conventional simplified ionchromatography system. Pump 42 is a conventional chromatography pumpwhich pumps the KOH output from generator 44 through sample injectionvalve 48 into chromatographic separator 50 packed with a chromatographicseparation medium, typically an ion exchange resin packed bed column.Alternatively, other forms of separation medium may be used such asporous hydrophobic chromatographic resin with essentially no permanentlyattached ion exchange sites.

[0065] In ion chromatography, the effluent from the separation column 50flows through suppressor 52 serving to suppress the conductivity of thebase and the effluent from separator 50, but not the conductivity of theions injected through sample injector 48. Then, the effluent fromsuppressor 52 is directed through a flow through detector 54, e.g. aconductivity detector, for detecting the resolved ions in the effluentfrom suppressor 52. A suitable data system, not shown as provided in theform of a conventional conductivity detector for measuring thesuppressor effluent in the conductivity cell in which the presence of anionic species produces an electrical signal proportional to itsconcentration. With the exception of generator 44, such ionchromatography systems are well known as illustrated in U.S. Pat. Nos.3,897,213; 3,920,397; 3,925,019; and 3,956,559 incorporated herein byreference.

[0066] Other forms of detectors 54 may also be employed and thesuppressor may be eliminated. Such other forms of detection include UV,fluorescence and electrochemical.

[0067] In the large capacity KOH generator, electrolysis reactionsproduce hydrogen and oxygen gases. When used in a chromatography system,the hydrogen gas, along with the KOH solution, is carried forward intothe chromatographic flow path. If hydrogen gas is produced in asignificant volume relative to the liquid flow, its presence can bedetrimental to the downstream chromatography process. The potentialproblem of hydrogen gas can be eliminated by application of Boyle's law.A flow restrictor can be placed after the detector flow cell to elevatethe pressure of the entire chromatography system. Under high pressure(e.g., 1000 psi or higher pressures), hydrogen gas is compressed to aninsignificant volume compared to the eluent flow so that it does notinterfere with the downstream chromatography process. This approachrequires the use of a detector flow cell capable of withstanding apressure of 1000 psi or more. In an ion chromatography system usingsuppressed conductivity detection, the above approach also requires theuse of a suppressor that is capable of withstanding a pressure of 1000psi or more. The necessary pressure to accomplish this depends on thevolume of gasses produced. However, for a typical system, a pressure ofat least 250 to 500 psi is sufficient. One mode of elevating thepressure is to connect a flow restrictor 56 such as a fine bore coiledtubing downstream of the detector (e.g. three meters of 0.005 in I.D.).This elevates the pressure throughout the chromatography system upstreamof the detector.

[0068] Another approach to eliminate the potential problem associatedwith hydrogen gas is to use an on-line pressure gas removal device toremove hydrogen gas from the KOH solution. FIG. 8 illustrates aschematic outline of an ion chromatography system employing a largecapacity KOH generator and an on-line high pressure gas removal device60 instead of flow restrictor 56 in FIG. 7. In this implementation, ahigh pressure gas removal device 60 is placed downstream of the outletof the large capacity KOH generator 44, suitably between it and sampleinjector 48. Hydrogen gas is effectively removed from the KOH eluentbefore it reaches the sample injector of the chromatography system sothat the downstream chromatographic process is not affected. Oneadvantage of this system is that a conventional detector flow cell andion chromatography suppressor can be used.

[0069] One preferred embodiment of the on-line high pressure gas removaldevice is shown in FIG. 9. In this embodiment, gas permeable polymerictubing 62 is used to remove hydrogen gas in the KOH product solutionunder high pressure. Aqueous solution 67 flows in an annular space 64outside of the gas permeable tubing 62 defined between tubing 64 andprotective tubing 66. The released hydrogen gas is removed from thedevice by in the flowing aqueous liquid stream in space 64 which alsoserves to prevent absorption of carbon dioxide from the ambient air intothe KOH product stream. One source of the aqueous liquid in space 64 isthe detector effluent.

[0070] Preferably, the polymeric tubing 62 is inert and has high burstpressure and high gas permeability. The inner volume of the gaspermeable tubing should be small so that it does not have large deadvolume and thus does not compromise the gradient performance of thelarge capacity eluent generator. It is preferred to use a gas permeabletubing with inside diameter less than 0.015 inch so that the gas removaldevice has low dead volume and high burst pressure.

[0071] The polymeric tubing prepared from a number of polymers includingpolymethylpentene, polypropylene, and fluoropolymers such as PTFE, ETFE,PFA, and FEP is gas permeable under high pressure and may be used as thegas removal tubing for the eluent generator.

[0072] The on-line high pressure gas removal device shown in FIG. 8 canalso be used to remove oxygen gas generated along with the acid solutionin a large capacity acid generator.

[0073] In another embodiment of the invention, not shown, the system ofFIG. 7 can be used in gradient ion or liquid chromatography where eluentcomponents in addition to KOH are required. A gradient pump, e.g. aDionex GP-40 pump type, can be used to deliver a prescribed mixture ofone or more eluent components from separate reservoirs to the highpressure KOH generation column. The eluent is modified with KOH which isgenerated on-line at the exit end of the KOH generation column. Theconcentration of KOH in the final eluent delivered to the separationcolumn can be controlled by controlling the applied current to the largecapacity KOH generator. The gradient system using the large capacity KOHgenerator is especially beneficial to applications that require the useof highly pure base hydroxide solution.

[0074] Referring to FIG. 10, another form of the present invention isillustrated. Here reservoir 10 includes a solution of cation saltsolution (e.g. one liter of K₂HPO₄ at 2 M concentration). Barrier 14extends substantially along the entire length of the mating sidegeneration chamber 16 in open communication with the interior of thechamber. Cathode 18 is in the form of a perforated platinum cathodewhich extends along the flow path of the aqueous stream through chamber12 in direct contact with beds of ion exchange resin 19 in K⁺ form onboth sides of cathode 18. Water flows through an inlet port, not shown,on the upstream side of the chamber. The KOH produced in chamber 12exits at an outlet port, not shown, at the downstream side of thechamber. The perforated platinum cathode is in the form of a screensuitably extending along the entire length of resin bed and isperforated to permit passage of solution through the cathode to ensurean efficient removal of KOH generated.

[0075] Another form of generation chamber 12 is illustrated in FIG. 11.This embodiment differs from that of FIG. 9 in the use of a cationexchange screen 70 in contact with perforated cathode 18 on one side andwith barrier 14 on the other side. The electrical path between anode 16and cathode 18 extends through barrier 14, cation exchange screen 10 andperforated cathode 18. The aqueous stream flows through the chamber 12inlet port, through perforated cathode 18 into cation exchange screen 70where it flows adjacent to the cathode and out the chamber 12 outlet onthe downstream side of screen 70.

[0076] In another embodiment of the generation chamber, not shown, theonly structural eluent within chamber 12 is cathode 18 in the form of aperforated platinum electrode screen in direct contact with barrier 14.The aqueous stream flows through the perforated platinum cathode screen.The screen uses openings of a size suitably on the order of 50-100 μm topermit the flow of the aqueous stream through the platinum screenwithout undue pressure drops. A suitable screen has a size of 1 to 5cm².

[0077] Another embodiment of the base generation chamber design isillustrated in FIG. 12. As in the embodiment of FIG. 10, barrier 14extends along the entire length of chamber 12. In this instance, theperforated platinum cathode 18 is sandwiched between non-charged screens72 and 74 suitably formed of a non-charged polymer such as apolypropylene which forms the fluid pathway in the generation chamber12. Screens 72 and 74 may be of the same size as the screen cathode inthe embodiment of FIG. 11. An inert lead, e.g. platinum wire 76,provides electrical contact with platinum cathode 18 and in directcontact with barrier 14. Upon the application of electrical current asmall amount of KOH is formed in situ. The KOH serves as the iontransport bridge between barrier 12 and platinum electrode 18. Screens72 and 74 have sufficient porosity to permit the flow of water throughthe screen without undue pressure drop.

[0078] The system of FIG. 12 can be operated by first filling chamber 12with KOH solution prepared externally which serves as the ion transportbridge between barrier 14 and cathode 18. Then current is applied. Goodcontact between the perforated disk-cathode 18 and barrier 14 may bemaintained by pressing one against the other. The electrode can extendacross all or part of the aqueous liquid flow path through the chamber12 to permit intimate contact with the flowing aqueous stream.

[0079] Other embodiments of the interior configuration of the basegeneration chamber may be employed so long as there is sufficientelectrical path between the anode and the cathode to permit the cationsto transport across the barrier and with the aqueous stream flowingthrough the chamber to permit the efficient generation of KOH. It hasbeen found that systems in which the cathode and a barrier in the formof a charged membrane extends substantially along the entire flow pathof the aqueous stream through the base generation chamber is veryefficient.

[0080] The system has been described with respect to generating a baseand specifically KOH. However, the system is also applicable to thegeneration of an acid by reversal of the polarity of the ion exchangebeds, barrier and the electrodes. In this instance, anion exchange beds,rather than cation exchange beds are employed. Also the barriers are ofa type which pass anions but not cations and block the flow of liquid.Suitable barriers for use in the production of acid can be prepared froma single or multiple ion exchange membrane of appropriate thickness or ablock or rod of ion exchange material. A suitable form of membrane issupplied by Membrane International of Glen Rock, N.J. (designatedAMI-7000 anion exchange membrane).

[0081] The cations or anions for use as the source in reservoir 10 mustalso be sufficiently water soluble in base or acid form to be used atthe desired concentrations. Suitable cations are metals, preferablyalkali metals such as sodium, potassium, lithium and cesium. Knownpacking for high capacity ion exchange resin beds provide such cationsor anion for use in the embodiment where resin is used as the source ofcations or anions. Typically, the resin support particles would be inthe potassium or sodium form. Potassium is a particularly effectiveexchangeable cation because of its high conductance. Suitable othercations are tetramethyl ammonium and tetraethyl ammonium. Analogously,suitable exchangeable anions for cation analysis include chloride,sulfate and methane sulfonate.

[0082] Using the concept described above, a large capacity acidgenerator can also be implemented. For example, a large capacitymethanesulfonic acid (MSA) generator employing a CH₃SO₃ ⁻ ion supplyreservoir is described here as an example. MSA generation chamber 12 ispacked with a strongly basic anion exchange resin in CH₃SO₃ ⁻ form andequipped with a Pt screen electrode (anode) which is in direct contactwith the anion exchange resin. The MSA generation chamber 12 isconnected to the CH₃SO₃ ⁻ ion supply reservoir 10 using one or moreanion ion exchange barriers of the same general type as barrier 14.Barrier 14 permits the passage of CH₃SO₃ ⁻ ions from the supplyreservoir into the resin bed in the MSA generation column, whileprecluding the passage of cations from the CH₃SO₃ ⁻ ion supply reservoirinto the MSA generation column. Barrier 14 also serves the role of ahigh pressure physical barrier that insulates the low pressure CH₃SO₃ ⁻ion supply compartment from the high pressure MSA generation chamber 12.

[0083] Analogous to the cation-source reservoir, the anion-source(CH₃SO₃ ⁻) reservoir 10 is equipped with a cathode and a gas vent hole.The reservoir (1 to 2 liters in volume) is filled with a solution of aMSA salt such as NH₄CH₃SO₃. The concentration of CH₃SO₃ ⁻ ions in thesolution is preferably 1 to 2 M or higher so that there is a sufficientamount of CH₃SO₃ ⁻ ions in the CH₃S0₃- ion supply reservoir for thegeneration of MSA over an extended period of time; however, the MSA saltsolution containing CH₃SO₃ ⁻ ions at lower concentrations can be used.It is preferred that the cation of the MSA salt used can not be reducedby the cathode int he CH₃SO₃ ⁻ ion supply reservoir. It is alsopreferred to use a “weakly basic cation” (e.g., NH₄ ⁺) defined to have abase dissociation constant (pK_(b)) of 4.5 or higher so that theconcentration of free OH⁻ ions in the solution is kept lower than 0.1mM. A “strongly basic cation” is defined to have a base dissociationconstant (pK_(b)) of less than 4.5. OH⁻ ions, like CH₃SO₃ ⁻ ions, canmigrate across the anion exchange connector into the MSA generationcolumn. If OH⁻ ions migrate across the anion exchange connector into theMSA generation column in significant amounts, the direct linearrelationship between the applied current and the concentration of MSAgenerated is lost because OH⁻ ions can combine with H⁺ ions generated atthe anode to form water, and thus the performance of the MSA generatoris compromised.

[0084] To operate the large capacity MSA system, deionized water ispumped through the MSA generation chamber 12, and a DC voltage isapplied between the anode is and cathode 18. Under the applied field,the electrolysis of water occurs at the anode and cathode. Water isreduced to form OH⁻ ions and hydrogen at the cathode:

2H₂O+2e⁻→2OH⁻+H₂↑  (4)

[0085] and oxidized to form H+ ions and oxygen at the anode:

H₂O+2e⁻→2H⁺+½O_(2↑)  (5)

[0086] CH₃SO₃ ⁻ ions migrate through barrier 14 into the resin bed inthe MSA generation chamber 12, and eventually combine with H⁺ ionsgenerated at the anode to produce a MSA solution suitable for use as ahigh purity eluent for ion or liquid chromatography.

[0087] The large capacity acid or base generator can also be implementedto generate high purity ion pairing reagents such as octanesulfonic acid(OSA) and tetrabutylammonium hydroxide (TBAOH) for use as eluents inmobile phase ion chromatography (MPIC) or reversed-phase ion pairchromatography (RPIPC).

[0088] Although much of the above discussion relates to use of thegenerated base or acid in ion and liquid chromatography, such use canalso be applied to other areas such as titration, flow injectionanalysis and post-column reactors.

[0089] Specifically the generated base can be used in combination with(a) conventional titration analyses, e.g. described in Douglas A. Skoogand Donald M. West, Fundamentals of Analytical Chemistry, 4th Edition,Saunders College Publishing, San Francisco, 1982, Chapter 8 Theory ofNeutralization, p. 195 or Douglas A. Skoog, Principles of InstrumentalAnalysis, 3rd Edition, Saunders College Publishing, San Francisco, 1985,Chapter 20 Potentiometric Methods, p. 638; (b) flow injection analysis,e.g., described in Theory and Automation, Skoog, Chapter 29, p. 858-859;and (c) post-column reactors, e.g. described in Paul R. Haddad and PeterE. Jackson, Ion Chromatography, Elsevier, N.Y., 1988, p. 387 and R. W.Frei Editor and K. Zech, Selective Sample Handling and Detection inHigh-Performance Liquid Chromatography, Elsevier, N.Y., 1988, p. 396.

[0090] The following examples are provided in order to furtherillustrate the present invention.

EXAMPLE 1

[0091] Generation of KOH using a KOH generator employing a largecapacity K⁺ ion supply reservoir (as illustrated in FIG. 2).

[0092] A large capacity KOH generator consisting of a K⁺ ion supplyreservoir 10 in the form of column (18-mm ID×185-mm length) and a KOHgeneration chamber in the form of column 12 (4-mm ID×30-mm length) wasconstructed.

[0093] The KOH generation chamber was packed with an 18-μm, 8%cross-link sulfonated styrene/divinyl benzene resin in K⁺ form. The K⁺ion supply column consisted of a 175 mm length bed of an 18 μm, 8%cross-link sulfonated styrene/divinyl benzene resin in K⁺ form and a 10mm length bed of a 50 μm polyacrylate resin in K⁺ form. The device wastested under an applied current of 30 mA and a carrier flow rate of 1.0mL/min for 48 hours. The conductance of the KOH solution generated andthe operating voltage of the KOH generator were monitored over thetesting period. The exhaustion profile (the conductance of the KOHsolution generated vs. time) and the operating voltage data are shown onFIG. 13. The device produced a constant output of KOH(18.7 mM KOH at thecarrier flow rate of 1.0 mL/min) for 44.4 hours, or a useful capacity of49.7 meq. After 44.4 hours of operation, the operating voltage increasedto 275 V (the operating voltage limit of the power supply used in theexperiment) due to the development of a less conductive neutralized zonein the weakly acidic carboxylated resin bed inside the K⁺ ion supplycolumn, and decreases in the operating current and concentration of KOHgenerated were observed. These results indicate the feasibility of usingthe large capacity KOH generator employing the large K⁺ ion supplycolumn to generate the KOH solution over an extended period of time.

EXAMPLE 2

[0094] Generation of KOH using a large capacity KOH generator employinga flow-through K⁺ ions supply column (as illustrated in FIG. 4).

[0095] A large capacity KOH generator employing the flow-through K⁺ ionsupply column was constructed to evaluate this embodiment of theinvention (FIG. 4). Both the flow-through K⁺ ion source reservoir 10 inthe form of column (4-mm ID×25-mm length) and the KOH generation chamber(4-mm ID×25-mm length) were packed with an 18 μm, 8% cross-linksulfonated styrene/divinyl benzene resin in K⁺ form and equipped withporous Pt frit electrodes at their outlets. A 100-mM KCI solution in aremote reservoir was pumped continuously through the flow-through K⁺ ionsupply column at a flow rate of 1.0 mL/min. The large capacity KOHgenerator was tested under applied currents of 10.5, 21, and 30.5 mA forabout 23 hours. The operating voltage ranged from 40 to 60 V during theexperiment. FIG. 14 shows the conductance profiles of the KOH solutionsgenerated at a carrier flow rate of 1.0 mL/min and applied currents of10.5, 21, and 30.5 mA. The concentration of KOH generated was directlyproportional to the applied current. The results indicate that it isfeasible to use a large capacity KOH generator employing a flow-throughK⁺ ion supply column to generate the KOH solution over an extendedperiod of time.

EXAMPLE 3

[0096] Generation of KOH using a large capacity generator employing a K⁺ion supply reservoir (as illustrated in FIG. 3).

[0097] A large capacity KOH generator employing a K⁺ ion sourcereservoir 10 was constructed to evaluate this preferred embodiment ofthe invention (FIG. 3). The KOH generation chamber (5.2-mm ID×37-mmlength) was packed with an 18-μm, 8% cross-link sulfonatedstyrene/divinyl benzene resin in K⁺ form and equipped with a porous Ptfrit electrode at its outlet. The K⁺ ion source reservoir 10 was filledwith a 2.0 M K₂HPO₄ solution. The large capacity KOH generator wasoperated continuously under a constant current of 30 mA and a carrierflow rate of 1.0 mL/min for a total of 832 hours. The operating voltagewas about 60 V during the test. The KOH solutions generated using thedevice were periodically collected and titrated using a 10-mM nitricacid standard to determine the concentration of KOH generated. FIG. 15shows the determined concentration of KOH in the solutions collected.Over the period of 744 hours, the average determined KOH concentrationwas 17.7 mM (n=18 and RSD=2.2%), corresponding to 95% of the theoreticalconcentration of 18.7 mM. The results indicate that it is feasible touse a large capacity KOH generator employing a large capacity K⁺ ionsupply reservoir to generate the KOH solution over an extended period oftime.

EXAMPLE 4

[0098] Generation of KOH using a large capacity generator employing a K⁺ion supply reservoir and three KOH generation chambers (as illustratedin FIG. 5).

[0099] A large capacity KOH generator employing a K⁺ ion supplyreservoir and three KOH generation chambers, as illustrated in FIG. 5,was constructed. Each KOH generation chamber (5.2-mm ID×10-mm length)was packed with an 18-μm, 8% cross-link sulfonated styrene/divinylbenzene resin in K⁺ form and equipped with a porous Pt frit electrode atits outlet. The K⁺ ion supply reservoir was filled with a 2.0 M K₂HPO₄solution. The large capacity KOH generator was used to generate KOHsolutions under applied currents ranging from 10 to 160 mA and carrierflow rates of 1.0 or 2.0 mL/min. The operating voltage for the KOHgenerator was 45 V when an applied current of 160 mA was maintained togenerate 50 mM KOH at 2.0 mL/minute.

[0100] The concentrations of KOH generated at different applied currentsusing the KOH generator were determined by titration using a 10-mMnitric acid standard. The results are summarized in Table 1. In this KOHgenerator, the KOH solution generated in the first KOH generationchamber flows through the second and third KOH generation chambers. Thepresence of KOH solution in the second and third KOH generation chambersdid not affect the KOH generation in the second and third chamber. Thepercent electrolytic yield of this KOH generator was very close to thetheoretical limit, ranging from 96.8 percent at 10 mA to 99.0 percent at100 mA, as shown in Table 1. There was also excellent correlation(R²=0.9998) between the applied current and the determined concentrationof KOH generated (FIG. 16). TABLE 1 Calculated and DeterminedConcentrations of KOH Generated Using a Large Capacity KOH Generatorwith Three KOH Generation Chambers Calculated Determined Applied Flowrate, Concentration, Concentration^(a), mM Percent Yield^(b) Percent RSDCurrent mL/min mM (n = 3) (n = 3) (n = 3)  10 mA 2.0 3.1 3.0 96.8 0.2 50 mA 2.0 15.5 15.1 97.4 0.4 100 mA 2.0 31.1 30.8 99.0 0.9 100 mA 1.062.2 61.2 98.4 0.5 30 mA + 10 mM 2.0 19.3 19.2 98.9 0.9 NaOH 60 mA + 10mM 2.0 28.7 28.4 98.4 1.3 NaOH

[0101] The above results indicate that connecting multiple KOHgeneration chambers in series is a viable approach to boost theconcentration of KOH generated. The results also demonstrate that KOH atrelatively high concentrations can be accurately generated using a largecapacity KOH generator with multiple KOH generation chambers withoutbeing limited by excessive heating.

EXAMPLE 5

[0102] Evaluation of a large capacity KOH generator employing a KOHgeneration chamber with multiple ion exchange connectors (as illustratedin FIG. 6).

[0103] A large capacity KOH generator employing a K⁺ ion sourcereservoir and a KOH generation chamber in the form of column with twomultiple ion exchange connectors, as illustrated in FIG. 7, wasconstructed. The K⁺ ion supply reservoir was filled with a 2.0 M K₂HPO₄solution. The KOH generation chamber 12 in the form of column ((5.2-mmID)×10-mm length) was packed with an 18-μm, 8% cross-link sulfonatedstyrene/divinyl benzene resin in K⁺ form and equipped with a porous Ptfrit electrode at its outlet. The KOH generation column was connected tothe K⁺ ion supply reservoir using either one or two ion exchangeconnectors (each with a 5 mm in contact diameter) during the experiment.The applied current was varied from 10 to 90 mA and the operatingvoltage was monitored. The carrier flow rate was maintained at 2.0mL/minute.

[0104] The dependence of the operating voltage on the applied currentdetermined for the KOH generator is shown in FIG. 17. For a givenapplied current, the operating voltages required for the generator usingtwo ion exchange connectors were about 30 percent lower than thoserequired for the generator using one ion exchange connector. The use ofmultiple ion exchange connectors in a single KOH generation columnclearly increases the pathway for the transport of K⁺ ions from the K⁺ion supply reservoir into the KOH generation column and thus reduces thedevice operating voltage. The results suggest that the use of multipleion exchange connectors in a single KOH generation column is a viableapproach to facilitate the generation of KOH at relatively highconcentrations.

EXAMPLE 6

[0105] Evaluation of different cathode configurations for the largecapacity KOH generator.

[0106] A large capacity KOH generator employing a K⁺ ion sourcereservoir, as illustrated in FIG. 3, was constructed. The K⁺ ion supplyreservoir was filled with a 2.0 M K₂HPO₄ solution. The KOH generationchamber in the form of column (5.2-mm ID×10-mm length) was packed withan 18 μm, 8% cross-link sulfonated styrene/divinyl benzene resin in K⁺form. The KOH generation column was connected to the K⁺ ion supplyreservoir using one ion exchange connector (5 mm in contact diameter).Three cathode configurations were tested for the KOH generation column:one porous Pt frit (4 mm diameter) placed at the outlet of thegeneration column, two porous Pt frits (4 mm diameter) placed at theinlet and outlet of the generation column, and a Pt screen that isformed to wrap around the resin bed in the KOH generation column. Theapplied current was varied from 1.0 to 70 mA and the operating voltagewas monitored. The carrier flow rate was maintained at 2.0 mL/minute.

[0107] The dependence of the operating voltage on the applied currentdetermined for the KOH generator operated in three cathodeconfigurations is shown in FIG. 18. At an applied current of 60 mA, theoperating voltage was 45 V when one porous Pt frit was used as thecathode, 40 V when two porous Pt frits were used as the cathodes, and 29V when the cathode was made of a Pt screen formed to wrap around theresin bed. The results indicate that the operating voltage of the KOHgenerator can be decreased significantly by increasing the contact areabetween the ion exchange resin and the electrode, so that KOH atrelatively high concentrations can be generated without being limited byexcessive heating.

EXAMPLE 7

[0108] On-line high pressure removal of hydrogen gas.

[0109] An on-line high pressure gas permeable removal device wasconstructed according to the design shown in FIG. 9. A polymeric tubing(0.020-inch OD×0.010-inch ID×1.0 meter length) obtained from BiogeneralInc. (San Diego, Calif.) was used as the gas permeable tubing in thedevice. The device was tested for removing hydrogen gas in the KOHsolution generated at applied currents up to 160 mA using the largecapacity KOH generator described in Example 4. The carrier flow rate forthe generator was 2.0 mL/minute. In some experiments, the outlet of thedevice was connected to a piece of 0.005-inch ID PEEK tubing thatgenerated a pressure drop of 1400 psi at 2.0 mL/min; the PEEK tubingoutlet was immersed in the deionized water in a small, clear glass vial,and the presence of hydrogen gas in the KOH solution was visuallymonitored (by observing the formation of gas bubbles). In someexperiments, the KOH generator and gas removal device were installed inan ion chromatography system as shown in FIG. 10, the baseline noise ofthe conductivity detector was monitored, and the flow of chromatographysystem effluent was used to shield the outside of the gas permeabletubing to remove the released hydrogen gas and prevent the readsorptionof carbon dioxide from the ambient air, as shown in FIG. 9.

[0110] The on-line high pressure gas removal device was highly effectivein removing the hydrogen gas. No hydrogen gas bubbles could be visuallyobserved in the KOH solution generated at applied currents up to 160 mA.FIG. 19 shows the baseline peak-to-peak noises measured at differentcurrents obtained using the device; they are similar to those obtainedwith the conventional ion chromatography system. At the applied currentof 160 mA, hydrogen gas is generated at a rate of about 1.1 mL/min (gasvolume at 14.7 psi). Therefore, the gas removal efficiency of the devicewas quite remarkable, especially considering the fact that the length oftubing used was only 1.0 meter and its internal volume was only 51 μL.

EXAMPLE 8

[0111] Use of a large capacity KOH generator in isocratic and gradientseparation of common anions by ion chromatography.

[0112] An ion chromatography system consisting of a large capacity KOHgenerator, an on-line high pressure gas removal device, and commonDionex ion chromatography system components was assembled as shown inFIG. 10. The large capacity KOH generator used was similar to the onedescribed in Example 3. The on-line high pressure gas removal devicedescribed in Example 7 was used. A Dionex AS 11 column (4-mm ID×250-mmlength) was used as the analytical separation column. In isocraticseparation experiments, the large capacity KOH generator was appliedwith a constant current of 40 mA to generate 12.4 mM KOH at 2.0mL/minute. In gradient separation experiments, the current applied tothe large capacity KOH generator was changed from 2.0 to 50 mA in stepsof 0.5 mA per 20 seconds to generate a gradient of KOH from 0.6 to 15.5mM at 2.0 mL/minute.

[0113]FIG. 20 and 21 show, respectively, the representative isocraticand gradient separation of fluoride, chloride, nitrate, sulfate, andphosphate. FIG. 22 shows the reproducible overlay of 16 consecutive KOHgradients generated using the large capacity KOH generator. It is worthyto point out that the chromatographic baseline shift during the KOHgradient was less than 50 nS in the chromatogram shown in FIG. 21. Ifthe same hydroxide gradient is generated using a conventional gradientpump, the baseline shift is usually about 500 to 1500 nS. These resultsdemonstrate that the high purity KOH solutions can be generatedreproducibly using the large capacity KOH generator, and usedeffectively as eluents in ion chromatography. The results also suggestthat the performance of an ion chromatography method can be enhancedbecause the use of high purity hydroxide solution generated on-lineresults in minimal baseline shifts during gradient separation, asillustrated in the next example.

EXAMPLE 9

[0114] Use of a large capacity KOH generator in determination of traceanions in high purity water by ion chromatography.

[0115] Dionex Application Note 113 describes a method for determinationof trace anions in high purity waters. In this method, the large volumedirection injection technique is used (sample loop is 750 μL), targetanions are separated on a Dionex microbore AS 11 column (2-mm ID×250-mmlength) using a NaOH gradient. FIG. 23 shows the typical chromatogramobtained when the NaOH gradient (0.5 to 26 mM NaOH) was generated usinga gradient pump and NaOH solutions prepared by conventional means. Thebaseline shift is about 500 nS during the gradient. The baseline shiftoccurs because NaOH solutions are easily contaminated with carbondioxide in the ambient air during the solution preparation and use, evenwith precautions.

[0116] To demonstrate the benefits of using high purity KOH eluentgenerated by the large capacity KOH generator, an ion chromatographysystem similar to the one used in Example 8 was assembled. A Dionexmicrobore AS- 11 column was used as the analytical separation column.The current applied to the large capacity KOH generator was changed from0.4 to 21 mA in steps of 0.4 mA per 17 seconds to generate a gradient ofKOH from 0.5 to 26 mM at 0.5 mL/minute.

[0117]FIG. 24 shows a representative chromatogram obtained for a sampleof deionized water spiked with 10 anions at levels of 0.9 to 3.0 ppb.Since the KOH solution generated with the large capacity KOH generatorwas essentially free of carbonate contamination, the observed baselineshift was less than 80 nS during the gradient. The significantly smallerbaseline shift during the gradient achieved using the KOH generatorleads to improvements in the method performance. These results suggestthat the performance of an ion chromatography method can be enhanced byusing a large capacity KOH generator.

EXAMPLE 10

[0118] Generation of methanesulfonic acid (MSA) using a large capacityMSA generator employing a large capacity CH₃SO₃ ⁻ ion supply reservoir.

[0119] A large capacity MSA generator employing a CH₃SO₃ ⁻ ion supplyreservoir was constructed to evaluate this preferred embodiment of theinvention. The MSA generation column (7-mm ID×10-mm length) was packedwith a 20-μm, 8% cross-link strongly basic (quaternary amine functionalgroups) styrene/divinyl benzene resin in CH₃SO₃ ⁻ form and equipped witha Pt screen cathode. The CH₃SO₃ ⁻ ion supply reservoir was filled with a2.0 M NH₄CH₃SO₃ solution. The large capacity MSA generator was used togenerate MSA solutions at applied currents ranging from 10 to 100 mA anda carrier flow rate of 1.0 or 2.0 mL/min. The operating voltage for thelarge capacity MSA generator was 9.5 V at 10 mA, 30 V at 50 mA, and 38.5V at 100 mA. The concentrations of MSA generated at 10, 40, and 80 mAwere determined by titration using a 10-mM NaOH standard. FIG. 25 showsthat there was excellent correlation (R²=0.9997) between the appliedcurrent and the determined concentration of MSA generated. In someexperiments, the current applied to the large capacity MSA generator waschanged from 28.5 mA to 70 mA in steps of 1.0 mA per 5 seconds togenerate a gradient of MSA from 17.7 mM to 43.5 mM at 1.0 mL/min. FIG.26 shows the reproducible overlay of 16 consecutive MSA gradientsgenerated using the large capacity MSA generator. These results indicatethat the large capacity MSA generator can be used to generate MSA atdesired concentrations accurately and reproducibly.

EXAMPLE 11

[0120] Use of the large capacity MSA generator in the separation ofcations by ion chromatography.

[0121] An ion chromatography system consisting of a large capacity MSAgenerator, an on-line high pressure gas removal device, and commonDionex ion chromatography system components was assembled. The largecapacity MSA generator described in Example 10 was used. The on-linehigh pressure gas removal device described in Example 7 was used. ADionex CS12A column (4-mm ID×250-mm length) was used as the analyticalseparation column. The current applied to the large capacity MSAgenerator was changed from 28.5 mA to 70 mA in steps of 1.0 mA per 5seconds to generate a gradient of MSA from 17.7 mM to 43.5 mM at 1.0mL/min. In some experiments, MSA gradients from 17.7 mM to 43.5 mM at1.0 mL/min were generated by using a Dionex GP40 gradient pump withdeionized water and a 100 mM MSA solution prepared from reagent gradeMSA.

[0122]FIG. 27 shows a representative gradient separation of 10 cationsusing the MSA gradient generated using the large capacity MSA generator.FIG. 28 shows the overlay of two representative chromatograms obtainedfor a high purity water sample spiked with 10 cations at sub to low μg/Llevels, using identical MSA gradients generated with either the largecapacity MSA generator or the GP40 gradient pump. The results show thatthe MSA generator gradient yielded lower detector background and smallerbaseline shift during the gradient than the GP40 pump gradient. Theseimprovements can be attributed to the fact that the MSA solutiongenerated using the large capacity MSA generated is of high purity andfree of contaminants that may be present in the reagent grade MSA.

[0123] The results also show that the elution of calcium, strontium, andbarium were delayed about one minute in the chromatogram obtained usingthe GP40 pump gradient when compared to the chromatogram obtained usingthe MSA generator gradient. In the ion chromatography system employingthe large capacity MSA generator and the on-line high pressure gasremoval device, the total dead volume of the two device was less than0.1 mL. On the other hand, the GP40 gradient pump used had a total deadvolume (consisted of dead volumes in proportioning valves and pumpheads) of about 1.0 mL. FIG. 29 shows the comparison of MSA gradientsgenerated using the large capacity MSA generator and the GP40 gradientpump. The results show that the profile of the MSA generator gradienthad minimal delay in the MSA gradient while noticeable gradient delaywas observed when the GP40 gradient pump was used.

What is claimed is:
 1. A method of generating a base comprising thesteps of: (a) providing a cation source in a cation source reservoir,(b) flowing an aqueous liquid stream through a first base generationchamber separated from said cation source reservoir by a first barriersubstantially preventing liquid flow while providing a cation transportbridge, (c) applying an electric potential between an anode inelectrical communication with said cation source reservoir and a cathodein electrical communication with said first base generation chamber toelectrolytically generate hydroxide ions in said first base generationchamber and to cause cations in said cation source reservoir toelectromigrate toward said first barrier and to be transported acrosssaid first barrier toward said cathode to combine with said transportedcations to form cation hydroxide, and (d) removing the cation hydroxidein an aqueous liquid stream as an effluent from said first basegeneration chamber.
 2. The method of claim 1 in which said cation sourcecomprises a cation-containing solution selected from the groupconsisting of a salt solution and a cation hydroxide solution.
 3. Themethod of claim 2 in which said cation-containing solution is suppliedto said cation source reservoir by pumping from a remote reservoir. 4.The method of claim 3 in which a stream of said cation-containingsolution is recycled from said cation reservoir to said remotereservoir.
 5. The method of claim I in which the volume of said cationsource reservoir is at least about 5 times the volume of said first basegeneration chamber.
 6. The method of claim I in which said first basegeneration chamber is pressurized and the pressure maintained in saidfirst base generation chamber is at least about 2 times any pressuremaintained in said cation source reservoir.
 7. The method of claim 1 inwhich said cation source comprises a cation exchange bed includingexchangeable cations of the type which form said cation hydroxide. 8.The method of claim 7 in which said cation exchange bed comprises cationexchange resin particles in a stationary bed or suspended in an aqueousliquid.
 9. The method of claim 7 in which said cation exchange bedincludes a downstream weakly acidic bed section proximal to said barrierand an upstream strongly acidic bed section distal to said firstbarrier, said upstream and downstream sections being in fluidcommunication, so that in the migration on the weakly acidic bed sectiontoward the cathode of hydronium ions generated at the anode is slowed incomparison to migration of the cations.
 10. The method of claim 7 inwhich a source of cation-containing solution is supplied to said cationreservoir by continuously pumping from a remote reservoir.
 11. Themethod of claim 10 in which a stream of said cation-containing solutionis recycled to said remote reservoir.
 12. The method of claim 1 in whichsaid cations in said cation source reservoir also electromigrate througha second barrier to said first base generation chamber.
 13. The methodof claim 1 including at least a second base generation chamber, and asecond barrier being disposed between said cation source reservoir andsaid base generation chamber.
 14. The method of claim 1 used to form abase eluent for an anion analysis system further comprising the stepsof: (e) flowing said cation hydroxide generated in step(d) and a liquidsample containing anions to be detected through a chromatographicseparator in which anions to be detected are chromatographicallyseparated, forming a chromatograph effluent, and (f) flowing saidchromatography effluent, with or without further treatment, past adetector in which the separated ions in said chromatography effluent aredetected.
 15. The method of claim 14 further comprising between steps(e) and (f) the step of: (g) flowing said chromatography effluentthrough a suppressor including a cation exchange bed to convert saidcation hydroxide to weakly ionized form, said chromatography effluentexisting as a suppressor effluent which flows past said detector. 16.The method of claim 15 further comprising, prior to step (e) thefollowing step: (h) pumping through a gradient pump one or more gradienteluents into said cation hydroxide eluent stream.
 17. The method ofclaim 14 further comprising pressurizing said chromatography effluent byflow through a pressure restrictor downstream from said chromatographyeffluent.
 18. A method of generating an acid comprising the steps of:(a) providing an anion source in an anion source reservoir, (b) flowingan aqueous liquid stream through a first acid generation chamberseparated from said anion source reservoir by a first barriersubstantially preventing liquid flow while providing an anion transportbridge, (c) applying an electric potential between a cathode inelectrical communication with said anion source reservoir and an anodein electrical communication with said first acid generation chamber toelectrolytically generate hydronium ions in said first acid generationchamber and to cause anions in said anion source reservoir toelectromigrate toward said first barrier and to be transported acrosssaid first barrier toward said anode to combine with said transportedanions to form an acid, and (d) removing the acid in an aqueous liquidstream as an effluent from said first acid generation chamber.
 19. Themethod of claim 18 in which said anion source comprises ananion-containing solution selected from the group consisting of a saltsolution and an acid solution.
 20. The method of claim 18 in which saidanion-containing solution is supplied to said anion source reservoir bypumping from a remote reservoir.
 21. The method of claim 20 in which astream of said anion-containing solution is recycled from said anionreservoir to said remote reservoir.
 22. The method of claim 18 in whichthe volume of said anion source reservoir is at least about 5 times thevolume of said first acid generation chamber.
 23. The method of claim 18in which said first acid generation chamber is pressurized and thepressure maintained in said first acid generation chamber is at leastabout 2 times any pressure maintained in said anion source reservoir.24. The method of claim 18 in which said anion source comprises an anionexchange bed including exchangeable anions of the type which form saidacid.
 25. The method of claim 24 in which said anion exchange bedcomprises an ion exchange resin particles in a stationary or suspendedin an aqueous liquid.
 26. The method of claim 24 in which said anionexchange bed includes a downstream weakly basic bed section proximal tosaid barrier and an upstream strongly basic bed section distal to saidfirst barrier, said upstream and downstream sections being in fluidcommunication, so that in the migration on the weakly basic bed sectiontoward the anode of hydroxide ions generated at the cathode is slowed incomparison to migration of the anions.
 27. The method of claim 24 inwhich a source of anion-containing solution is supplied to said anionreservoir by continuously pumping from a remote reservoir.
 28. Themethod of claim 27 in which a stream of said anion-containing solutionis recycled to said remote reservoir.
 29. The method of claim 18 inwhich said anions in said anion source reservoir also electromigratethrough a second barrier to said first acid generation chamber.
 30. Themethod of claim 18 including at least a second anion generation chamber,and a second barrier being disposed between said anion source reservoirand said anion generation chamber.
 31. The method of claim 18 used toform an acid eluent for an cation analysis system further comprising thesteps of: (e) flowing said acid generated in step(d) and a liquid samplecontaining cations to be detected through a chromatographic separator inwhich cations to be detected are chromatographically separated, forminga chromatograph effluent, and (f) flowing said chromatography effluent,with or without further treatment, past a detector in which theseparated cations in said, chromatography effluent are detected.
 32. Themethod of claim 31 further comprising between steps (e) and (f) the stepof: (g) flowing said chromatography effluent through a suppressorincluding an anion exchange bed to convert said acid to weakly ionizedform, said chromatography effluent existing as a suppressor effluentwhich flows past said detector.
 33. The method of claim 32 furthercomprising, prior to step (c) the following step: (h) pumping through agradient pump one or more gradient eluents into said acid eluent stream.34. The method of claim 31 further comprising pressurizing saidchromatography effluent by flow through a pressure restrictor downstreamfrom said chromatography effluent.
 35. An apparatus for generating anacid or base comprising: (a) an ion source reservoir containing a sourceof either anions or cations, (b) an acid or base generation chamberhaving inlet and outlet ports, (c) a charged first barrier disposedbetween said ion source reservoir and said acid or base generationchamber, said barrier substantially preventing liquid flow whileproviding an ion transport bridge for only ions of one charge, positiveor negative, (d) a first electrode in electrical communication with saidion source reservoir, (e) a second electrode in electrical communicationwith said first acid or base generation chamber, and (f) an aqueousliquid source in fluid communication with said acid or base generationchamber inlet port.
 36. The apparatus of claim 35 further comprising:(g) a power supply for applying an electrical potential between saidfirst and second electrodes.
 37. The apparatus of claim 36 in which saidacid or base generated in said acid or base generation chamber is usedas an eluent stream for analysis of ions of interest of one charge only,positive or negative, said apparatus further comprising: (h) a sampleinjection port for injecting a liquid sample stream of ions to bedetected, (i) a chromatographic separator for separating said ions ofinterest, and having inlet and outlet ports, said inlet port being influid communication with said sample injection port and said acid orbase generator outlet port, whereby a chromatography effluent exits fromsaid outlet port, and (j) a detector in fluid communication with saidchromatographic separator for detecting the separated ions of interestin said chromatography effluent.
 38. The apparatus of claim 37 furthercomprising: (k) a gradient pump for pumping one or more gradient eluentsinto said ion-containing solution generated in said first acid or basegeneration chamber.
 39. The apparatus of claim 37 further comprising:(k) a flow restrictor in fluid communication with the outlet of saidfirst acid or base generation chamber outlet port.
 40. The apparatus ofclaim 35 in which said ion source reservoir has inlet and outlet ports,said apparatus further comprising: (g) a remote reservoir forion-containing solution having inlet and outlet ports, and (h) a pumpfor pumping ion-containing solution from said remote reservoir outletport to said ion source reservoir inlet port.
 41. The apparatus of claim40 further comprising: (i) a recycle conduit connecting said ion sourcereservoir outlet port and said remote reservoir inlet port.
 42. Theapparatus of claim 33 in which the volume of said ion source reservoiris at least 5 times the volume of said acid or base generator chamber.43. The apparatus of claim 35 in which said cation source comprises anion exchange bed including exchangeable ions of the type which form saidacid or base.
 44. The apparatus of claim 43 in which said ion exchangebed comprises a stationary bed of ion exchange resin particles or resinparticles suspended in an aqueous liquid.
 45. The apparatus of claim 43in which said ion exchange bed comprises a bed of ion exchange resinparticles including a downstream weakly acidic or basic section proximalto said first barrier and an upstream strongly acidic or strongly basicsection of the same charge as said weakly acidic or weakly basic sectionand in fluid communication therewith.
 46. The apparatus of claim 35further comprising a second barrier of the same type as said firstbarrier disposed between said ion source reservoir and said first acidor base generation chamber.
 47. The apparatus of claim 35 furthercomprising: (g) a bed of ion exchange resin with exchangeable ions ofthe same charge as said first barrier disposed in said generationchamber between said first barrier and said second electrode andproviding an ion path therebetween.
 48. The apparatus of claim 35further comprising: (g) a charged screen of the same charge as saidfirst barrier disposed between said first barrier and said secondelectrode in said generation chamber and providing an ion paththerebetween.
 49. The apparatus of claim 35 further comprising anuncharged screen between said first barrier and said second electrode insaid generation chamber.